Day20.java

public class Day20 {

	private final static int TARGET = 29000000;
	private final static int MAX = 1000000;
	static int[] presents = new int[MAX];

	public static void main(String[] args) {

		// Part 1
		solve(false);
		int sought = findHouse();
		System.out.println("Part 1: House number: " + sought);

		// Part 2
		solve(true);
		sought = findHouse();
		System.out.println("Part 2: House number: " + sought);

	}

	public static void init() {
		for (int i = 0; i < MAX; i++) {
			presents[i] = 0;
		}
	}

	public static int findHouse() {
		for (int house = 1; house < MAX; house++) {
			if (presents[house] >= TARGET) {
				return house;
			}
		}
		return -1; 
	}

	public static void solve(boolean part2) {
		init();
		int multiplier = (part2 ? 11 : 10);
		for (int elf = 1; elf < MAX; elf++) {
			for (int house = elf; (house < MAX && (!part2 || (house <= elf * 50))); house += elf) {
				presents[house] += (elf * multiplier);
			}

		}

	}

}